1. Field of the Invention
The present invention relates generally to a sparkplug disposed in an internal combustion engine, and more particularly to the spark plug having a long spark discharging gap and a firing area of electrodes largely protruded into a combustion chamber of the engine.
2. Description of Related Art
In an internal combustion engine, a cooling performance has been heightened by improving the arrangement of water jackets disposed in an engine head. Therefore, a structure of an engine head is complicated, and a space for fitting a spark plug to a cylinder head of the engine is narrowed.
In this case, it is required to lessen the diameter of a male thread of the spark plug to be engaged with a female thread of the head. However, when the diameter of the thread is lessened, a top portion of an insulator disposed on an outer circumferential surface of a center electrode is thinned so as to undesirably cause dielectric breakdown in the plug, and a pocket bore formed between the insulator and a housing connected with a ground electrode is narrowed so as to cause transverse flying sparks. In the transverse flying sparks, sparks are undesirably discharged from the center electrode to an end portion of the housing through a surface portion of the insulator.
To solve these problems, Published Japanese Patent First Publication No. 2000-243535 discloses a spark plug having an attaching screw engaged with a cylinder head. The diameter of the screw is equal to or smaller than 12 mm. In this plug, the thickness of a top portion of an insulator is set to be equal to or larger than 1.1 mm to heighten a withstand voltage of the insulator, and the diameter of a center electrode is lessened so as to widen a pocket bore for the purpose of preventing a phenomenon of transverse flying sparks.
However, when this plug disclosed in the Publication is used for a long period of time, the tips of the center and ground electrodes facing each other are melted and partially lost due to sparks discharged between the electrode tips. Therefore, a spark discharging gap between the tips is lengthened. This lengthened spark discharging gap easily induces transverse flying sparks. Therefore, although the plug having a structure disclosed in the Publication can prevent transverse flying sparks when being used for a comparatively short period of time, transverse flying sparks can easily occur in the plug when sparks are discharged between the tips of the center and ground electrodes for a long period of time so as to lose the tips of the electrodes. That is, when a spark plug is used for its original purpose for a long period of time, the plug loses its original function.
Further, low fuel economy and low emission have recently been required, so that a higher ignition performance is desired in a spark plug. To realize the higher ignition performance, an extension type spark plug having a wide spark discharging gap is required. In the extension type spark plug, the tips of the center and ground electrodes are largely protruded from a cylinder head into a combustion chamber of an engine so as to place a firing area between the tips in the center of the combustion chamber. When a wide spark discharging gap type plug has a structure disclosed in the Publication No. 2000-243535, not only the plug can easily cause transverse flying sparks, but also it is difficult to excessively protrude the chips toward the combustion chamber for the purpose of preventing the pre-ignition.
Moreover, the extension type spark plug has a long ground electrode extended from a metallic housing. When sparks are discharged between tips of center and ground electrodes, it is difficult to transfer heat received in the tip of the ground electrode to the housing. Therefore, the tip of the ground electrode is easily heated up to a high temperature, so that the tip may be undesirably melted or oxidized by a gas of the chamber.